This invention relates to spectrophotometer and more particularly to ratiometric type spectrophotometers.
A ratiometric spectrophotometer system is one which takes the ratio between the sample and the reference beams, as contrasted to an optical null system, which takes and adjusts a mechanical optical attenuator in and out of the reference beam to match the radiation through the sample area. In the ratiometric spectrometer system there are two clear optical paths and they come together and impinge on one detector. A chopper is provided so that the detector sees the reference, the sample and a blank area, which is the background radiation in the system. That is, there is a comparison of those three levels, the background radiation, the sample radiation and the reference radiation.
Now what happens is that the sample radiation is taken and the background radiation is subtracted from it to give an absolute amplitude. Then the background radiation is subtracted from the reference radiation to obtain a corrected reference, which is divided into the corrected sample radiation to obtain the required ratio of sample to reference transmission.
Heretofore, attempts have been made to try to move various elements in the spectrophotometer in unison by means of cams, but difficulties were experienced. Stop-start repeatability could not be obtained because of the inertia involved. In addition, it was difficult and time consuming to accurately fabricate the cam surfaces.
Heretofore problems were caused by so called "uncompensation". Uncompensation comes about in monochromators because the sample looks at one wavelength of information and the reference looks at a second piece of information which is not at the same wavelength. As a result, when there are real sharp bands like in carbon dioxide or water vapor bands they pick up unequal results and it looks like noise. That is, observation is made high on the side of a sharp end for the reference and low for the sample so that the two readings are not functionally what one would like to know.
In summary, a problem with the ratiometric spectrophotometer resides in the fact that, if the grating is in motion while spectral information is being gathered, a significant degradation in the resultant signal occurs.